1. Field of the Invention
The present invention concerns an apparatus for controlling an antenna arrangement in a magnetic resonance apparatus, wherein the antenna arrangement is of the type having two feed points that respectively feed in two partial signals, the two feed points being arranged on a cross-section of the antenna arrangement that has a center point, with a first of the feed points forming a first angle with the center point relative to a horizontal axis of the cross-section, and the second of the feed points forming a second angle with the center point relative to the horizontal axis of the cross section.
2. Description of the Prior Art
In magnetic resonance apparatuses with magnetic field strength of greater than/equal to three Tesla, considerable eddy currents can be induced in the body of a patient to be examined.
In such apparatuses, the homogeneous B1 magnetic field (RF field) of the antenna arrangement used for magnetic resonance examination is affected by the presence of the patient's body so that inhomogeneities occur in the B1 magnetic field. In specific cases this leads to problems in the evaluation of measurement results and/or to problems in the imaging of specific body regions.
In conventional magnetic resonance systems, a field distribution is achieved with the antenna arrangement used for examination, wherein the antenna arrangement for whole-body examinations is fashioned as a birdcage antenna, for example.
FIG. 3 shows an exemplary arrangement to feed an antenna arrangement ANT according to the prior art.
The antenna arrangement ANT is designed as a cylindrical birdcage antenna and possesses two feed points A1x and A2x as well as eight longitudinal rods LS1 through LS8 shown here.
The two ends of the longitudinal rods LS1 through LS8 are respectively connected with one another via circular termination rings AR1 and AR2.
A device SPLIT1 for signal splitting possesses two outputs OUT11, OUT21 as well as an input IN11. an amplified radio-frequency magnetic resonance signal VSS, obtained by amplification in a pre-amplifier PA of a detected signal SS, is connected to the input IN11 of the device SPLIT1 for signal splitting. This radio-frequency magnetic resonance signal VSS is divided by the device SPLIT1 for signal splitting into two partial signals SS11 and SS21 of equal amplitude that possess a phase difference of 90° relative to one another.
A first partial signal SS11 is therefore present at a first output OUT11 of the device SPLIT1 for signal splitting while a second partial signal SS21 that is phase-shifted relative to this first partial signal SS11 is present at a second output OUT21.
An additional input IN21 of the device SPLIT1 for signal splitting is connected via a terminating resistor Z1 with a compensation potential (ground) in which the power reflected by the antenna is absorbed.
Each of the two outputs OUT11, OUT21 is connected with precisely one feed point A1x, A2x of the antenna arrangement ANT so that the antenna arrangement ANT develops what is known as a circularly polarized magnetic field via the partial signals SS11, SS21.
FIG. 3 also shows two circular cross-sections QS1, QS2 of the cylindrical antenna arrangement ANT. The two cross-sections QS1, QS2 differ in the spatial arrangement of the two feed points A1x and A2x. 
The two feed points A1x, A2x form a right triangle with a center point M of the circular cross-section. The right angle of this right triangle is present at the center point M.
In a circular cross-section QS1, a first feed point A11 is arranged at an angle α11 of −90° relative to a horizontal axis of the circle or, respectively, the circular cross-section QS1. A second feed point A21 is arranged at an angle α22 of 0° relative to the horizontal axis of the circle or, respectively, of the circular cross-section QS1.
This arrangement of the two feed points A11, A21 causes the radiating elements of the antenna arrangement to be mutually decoupled, and this decoupling is not disrupted by the presence of a patient.
In the circular cross-section QS2, a first feed point A12 is arranged at an angle α11 of −135° relative to a horizontal axis of the circle, or of the circular cross-section QS2. A second feed point A22 is arranged at an angle α22 of −45° relative to the horizontal axis of the circle or, respectively, of the circular cross-section QS2.
This arrangement of the two feed points A12, A22 causes the radiating elements of the antenna arrangement to be uniformly loaded by the body of the patient.
With reference to FIG. 3, FIG. 4 shows an additional exemplary arrangement for feeding an antenna arrangement ANT according to the prior art.
Each of the two outputs OUT11, OUT21 is connected via respective adjustment devices EV1, EV2 with precisely one of the feed points A1, A2 of the antenna arrangement ANT.
With the use of the adjustment devices EV1, EV2, the two partial signals SS11 and SS21 are varied in their phase difference relative to one another in order to form a cross-polarized magnetic field.
Furthermore, it is known in the prior art to control two inputs or feed points of an antenna arrangement with partial signals such that the antenna arrangement forms not a circularly polarized magnetic field, but rather an elliptically polarized magnetic field.
For this purpose, additional components are necessary which help the two partial signals exhibit unequal amplitudes and phase positions and arrive in this way at the antenna arrangement.
As described above, in these apparatuses the antenna arrangement that is used is affected by the real patient body such that inhomogeneities in the B1 field occur that in turn lead to disruptions in the imaging of specific body regions.
It should be noted that, although a magnetic field within an empty antenna arrangement is elliptically polarized, a circularly polarized magnetic field is formed in the patient by interaction between the patient and the elliptical field.